Ballasts for gas discharge lamps provide high ignition voltages for starting the lamps. The ignition voltages supplied by preheat type ballasts are typically on the order of several hundred volts (e.g., 500 volts peak), while those provided by instant-start type ballasts may exceed 1000 volts peak. As a consequence of these high ignition voltages, ballasts are subject to the problem of output arcing.
Output arcing may occur in any of a number of different ways. For example, in fluorescent lighting installations, it is a common practice to replace failed lamps while AC power is applied to the ballast. This practice is referred to as “live” relamping. During live relamping, as a lamp is being removed or inserted, a momentary arc may form between the fixture socket contacts and a pin of the lamp. As another example, a sustained arc (as opposed to a momentary arc) arc may occur due to poor connections in the output wiring or the lamp sockets, or if a lamp is improperly installed in such a way that a small gap exists between the lamp pins and the contacts within the fixture sockets. If a connection to the lamp(s) is compromised due to a defective lamp socket or defective wiring, a high intensity, high temperature arc may be produced across the air gap caused by these faulty conditions.
Arcing is generally acknowledged to cause degradation of the contacts in the fixture sockets and undue stress on components within the ballast. Sustained arcing is especially undesirable because of its tendency to produce potentially destructive heating. In order to minimize any ill effects due to arcing, it is important that the arc be promptly extinguished. This requires a ballast that is capable of quickly detecting an arc and, subsequently, taking appropriate action to quickly extinguish the arc.
It also important that arc detection be inhibited during certain periods, such as inverter startup and lamp ignition. For instance, the normal starting process of the lamp is generally accompanied by the same types of electrical disturbances that occur during output arcing. Thus, unless arc detection is inhibited during lamp starting, the ballast may be prevented from properly igniting the lamp. Additionally, although most lamps are capable, under ideal conditions, of igniting and operating normally within a short period of time (e.g., 20 milliseconds), some lamps, due to age or low temperature, require a much longer time to ignite and stabilize. Thus, arc detection should be inhibited for a period that is long enough (e.g., 200 milliseconds) to accommodate lamp starting under conditions that are less than ideal.
It is also desirable that a ballast possess some type of automatic restart capability wherein, within a specified time following detection of an arc and shutdown of the ballast, periodic attempts are made to restart the ballast and ignite the lamp. This feature is desirable in order to prevent a “latched” shutdown of the ballast (which necessitates that power to the ballast be turned off and then on again in order to reset the ballast) in the event of false detection due to a momentary power line transient or any of a number of anomalous phenomena that pose no real threat to ballast reliability or safety. Also, because lamps are somewhat unpredictable, it is possible that an otherwise “good” lamp may sometimes fail to properly start on the first attempt. In such a case, a ballast with automatic restart capability will periodically attempt to start the lamp, rather than simply latching the ballast in a shutdown state until the power to the ballast is cycled. As long as a condition that causes arcing is present, the ballast may cause the lamp to periodically flash (e.g., the lamp will light for up to 200 milliseconds every 2 seconds), thereby provide a useful visual indication that the fixture and/or lamp requires servicing.
Thus, a need exists for a ballast having an arc protection circuit that quickly detects an output arc and that then takes appropriate action to quickly extinguish the arc. A need also exists for a ballast having an arc protection circuit that provides an inhibit period in order to allow for proper lamp starting. A further need exists for a ballast having an arc protection circuit that provides automatic restart capability in order to accommodate false detection and anomalous starting failure of a “good” lamp, as well as to provide a useful visual indication of a need for servicing the lamp(s) and/or fixture. A further need exists for a ballast having an protection circuit that is economical and readily implemented within existing ballasts. Such a ballast and arc protection circuit would represent a considerable advance over the prior art.